This disclosure relates to powder metallurgy. In particular, this disclosure relates to powder metal formulations for powder metallurgy.
Powder metallurgy is an alternative to more traditional metal forming techniques such as casting. Using powder metallurgy, parts with complex geometries may be fabricated that have dimensions very close to those dimensions desired in the final part. This dimensional accuracy can save significant expense in machining or reworking, particularly for parts having large production volumes.
Parts made by powder metallurgy are typically formed in the following way. First, a formulation including one or more powder metals and a lubricant material is compacted in a tool and die set under pressure to form a PM compact. This PM compact is then heated to remove the lubricant material and to sinter the individual particles of the powder metal together by diffusion-based mass transport. Sintering is typically performed by heating the powder metal material to a temperature that is either slightly below or above its solidus temperature. When held below the solidus sintering occurs in the absence of a liquid phase. This is commonly referred to as solid state sintering. When held above the solidus, a controlled fraction of liquid phase is formed. Sintering in this manner is known as liquid phase sintering. Regardless of the sintering temperature employed, the sintered part is very similar in shape to the original compact.
During the sintering process, it is common for the parts to shrink dimensionally. As diffusion occurs, adjacent particles will neck into one another to form permanent bonds with one another and to begin to fill any voids between the particles. This densification closes and/or decreases the size of the pores and decreases the overall size of the sintered part in comparison to the compact. Even at long sintering times, however, some voids will remain in the sintered part. Unfortunately, for sintered parts that are less than fully dense, the mechanical strength of those sintered parts are also usually somewhat less than that of a wrought part.
Moreover, in many sintered parts, the shrinkage may be, for any of a number of reasons, different in various directions. This kind of anisotropic shrinkage can alter the dimensional accuracy of the as-sintered final part and, in some cases, may require that parts be reworked after sintering.
Hence, a need exists for improved powder metals. In particular, there has been a continued need for powder metals that, when sintered, have mechanical strength approaching that of their wrought counterparts and that do not exhibit anisotropic shrinkage.